The emerald-cut diamond has aesthetics and visual qualities that make it preferable to brilliant-cut diamonds for certain applications or buyers. There are many angles and dimensions that can be selected by the stone cutter to achieve the desired emerald cut. Commonly, cutters select parameters to maximize stone weight and, usually to a lesser priority, minimizing the deductions for the cut and/or light performance defects. Thus, the popular emerald-cut parameters have been standardized for maximizing stone weight, and as a result it can be difficult to obtain good light performance under these guidelines.
The symmetrical emerald cut is exemplified according to embodiments of the present invention in FIGS. 1, 2, 3A, and 3B, which show a crown C and pavilion P above and below a girdle G. The crown C has a number of steps called “tiers” designated, e.g., as crown first tier C1, crown second tier C2, crown third tier C3, and possibly one or more additional crown tiers (not shown) as well as a table T. Similarly, the pavilion P may have a pavilion first tier P1, pavilion second tier P2, pavilion third tier P3 (FIGS. 1, 2, 3A), and possibly a fourth tier P4 (FIG. 3B), or more additional pavilion tiers (not shown) that terminate at an edge or culet CT. Three (FIGS. 1, 2, 3A), and to a lesser extent four (FIG. 3B), pavilion tiers are common. The first tiers C1, P1 adjacent the girdle G are also referred to as the crown or pavilion main facets, or simply as “mains.” FIG. 3B is a side sectional view of an emerald cut dismond with a 4-tier pavilion
The cut also has a variable ratio of length L to width W, both measured at the girdle, that can vary from 1 for square and octagonal emerald cuts, to a higher ratio for rectangular emerald cuts, and a variable corner ratio CR, i.e., the ratio of the corner width to the width W, as measured along the width W. High L/W ratios define a generally rectangular stone that is elongated, whereas a low L/W close to 1 will have a squarer appearance. Similarly, high corner ratios CR produce a more octagonal stone, whereas a low CR is more square or rectangular.
Other dimensions include the ratio or percentage of the width WT of the table T to the width W of the stone at the girdle, often called the “table percentage”; the overall depth of the stone or the depth of the crown C or pavilion P, reported as a percentage of the width W; and the height of the girdle G, which is also given as a percentage of the width W.
The widths of the crown tiers C1, C2, C3, and so on are normally substantially equivalent, as are the widths of the pavilion tiers P1, P2, P3, P4, and so on. The angles of these facets are normally given as the angle with respect to the table T. Often the angles of each of the tiers is given, and sometimes the crown and pavilion mains C1, P1 are given along with the differences in the angles or “breaks” between the adjacent tiers. E.g., a stone with crown angles for C1, C2, C3 of 35°, 28°, 21° may be referred to as having a 35° crown main angle and crown breaks of 7° (T1 to T2) and 7° (T2 to T3), or in shorthand as simply 7-7; while a stone with pavilion angles for P1, P2, P3 of 54°, 43°, 36° may be referred to as having a 54° pavilion main angle and pavilion breaks of 11° (P1 to P2) and 7.5° (P2 to P3), or in shorthand as simply 11-7.5.
Emerald-cut diamonds are typically cut for visual appearance of the stone shape and maximum stone weight, and thus the crown and pavilion main angles C1, P1 may be relatively steep, the breaks large, the table percentages (100WT/W) high, etc. The popular parameters have been developed more or less along these lines, and different cutters have their own preferences. For example, some jewelers may provide general cut guidelines such as the following:
TABLE 1Typical Emerald-cut GuidelinesRating:ExcellentVery GoodGoodFairPoorTable (% W)61-6957-60 or 70-7254-56 or 73-7451-53 or 75-79<51 or >79Depth (% W)61-6759-60.9 or57-58.9 or54-56.9 or<54 or >7967.1-7070.1-7474.1-79GirdleV. Thin-S. ThickThickV. ThickEx. Thin or Ex.ThickCuletNoneV. SmallSmallMedium>MediumL/W -1.4-1.51.30-1.39 or1.20-1.29 or1.15-1.19 or<1.15 or >1.90Rectangle1.51-1.601.61-1.801.81-1.90
More recently, it has been possible to take light performance into account. For example, in 2004 the American Gem Society Laboratories (“AGSL” or “AGS”) developed the angular spectrum evaluation tool (ASET®) imaging disclosed in U.S. Pat. No. 7,355,683, hereby incorporated herein by reference, as a way to determine how a diamond is handling and returning light to the observer. ASET images are generated by illuminating the diamond with green light from the sides in the upper hemisphere up to 45 degrees from the horizontal plane of the table facet, representing low-intensity light from an indirect source; red light from above at 45 to 75 degrees from horizontal, representing the brightest, high-intensity light; and blue light from above at 75 to 90 degrees, representing areas of obstruction, i.e., light that the diamond cannot take in due to the body of the observer, which the viewer sees as contrast. Using ASET imaging, the AGS can mathematically calculate the light performance grade for a virtual diamond with a given set of proportions, e.g., the cutter can confirm that the proportions proposed for cutting the diamond should obtain an AGS light performance grade of 0, i.e., light performance deductions equal to or less than 0.5. The procedure is described in Yantzer, Peter et al., “Foundation, Research Results and Application of the New AGS Cut Grading System”, American Gem Society (2005), published at cdn.ymaws.com/www.americangemsociety.org/resource/resmgr/docs/AGSLab/AGS-Cut-System.pdf. Using ASET imaging, as well as ray tracing and virtual diamond modeling software, for some popular cuts and conventional facet angles, the AGS have developed ASET grade maps that show the theoretical light performance deductions or grades for virtual diamonds with perfect polish and symmetry, as a function of the various facet angles that can be selected by the cutter.
In using these maps, however, the process for planning and executing the cutting of a diamond may take the final weight of the diamond into account as a primary target, and thus the diamantaire seeking a high light performance may frequently select the steepest angles and largest depths possible, or sacrifice light performance for a heavier stone.
Even with these tools and light performance as a primary objective, however, it can be difficult to obtain an emerald-cut with the better light performance grades (AGS 0 or 1). For starters, both the polish and symmetry of the facets must be Ideal (AGS) or at least Excellent (AGS). Then, the facet angles that achieve the AGS 0 or 1 grades must be selected, and the diamond cut and polished according to the selected parameters. For example, the AGS has determined the combinations for emerald-cut stones with common crown breaks of 7-7 and common pavilion breaks of 11-7.5 over a range of L/W ratios, corner ratios, table percentages, crown main angles, and pavilion main angles, that are needed to obtain Ideal cut proportions for maximum light return or brilliance. AGS has made available the results in the Emerald Cut Guidelines (2006), published at americangemsociety.org/page/emeraldguidelines. Unfortunately, many of the 0-grade combinations of crown and pavilion mains shown on these emerald-cut maps are isolated from combinations of similar crown and pavilion angles, meaning that the cutter has limited flexibility in the selection of angles, and the angles must be cut very precisely to obtain the best light performance grades.
For example, FIG. 4 shows a portion of the grade map for an old time emerald-cut with crown breaks of 5.5-2.5 and pavilion breaks of 11.5-4.5 for pavilion mains from 51.0-55.0 and crown mains of 39.0-41.0. It is seen that the 0-grade crown/pavilion mains combination domains are not continuous and a 0 grade will not obtain unless the crown and/or pavilion mains are precisely within ranges of 0.5 to 1.0 degrees, i.e., pavilion mains 53.5-54.5 for crown main of 39.0-39.5 degrees, or crown main of 40.0-41.0 for pavilion main of 53.5. The corresponding ASET image for the old time emerald-cut of FIG. 4 for combination 100 of a crown main angle of 40.0 and a pavilion main angle of 53.5 is seen in FIG. 5.
As additional examples, similarly isolated “island” areas of 0-grade domains are seen in FIGS. 6 and 7 for the AGS emerald-cut guidelines for tables of 55% (FIG. 6) and 60% (FIG. 7), L/W ratio of 1.4, corner ratio of 14%, crown breaks 7-7, pavilion breaks 11-7, and 3% girdles.
What is needed is a set of emerald-cut parameters that can more easily obtain Excellent and/or Ideal light performance over wider, more continuous ranges of crown and pavilion main angle combinations, and/or that would give the cutter more flexibility in the selection, and/or leeway in the accuracy, of the angles or other cutting parameters.